Reducing electroless silver plating solution and reducing electroless silver plating method

ABSTRACT

Provided are a reducing electroless silver plating solution and a reducing electroless silver plating method using the silver plating solution, the reducing electroless silver plating solution being capable of preventing decomposition of silver in the plating solution thereby to maintain stability of the solution and also being capable of preventing excessive roughening of an underlying metal or the like thereby to form a plating film having good film characteristics and a good appearance. The reducing electroless silver plating solution according to the present invention comprises a water-soluble silver salt and a reducing agent, wherein cyanide ions in a concentration of 0.006×10 −3  mol/L to 12.5×10 −3  mol/L are contained.

FIELD OF THE INVENTION

The present invention relates to a reducing electroless silver plating solution and a reducing electroless silver plating method, more specifically, relates to a reducing electroless silver plating solution and a reducing electroless silver plating method using the silver plating solution, the reducing electroless silver plating solution being stable and being capable of forming a good plating film without excessive roughening of an underlying metal or the like.

The present application asserts priority rights based on JP Patent Application 2011-235559 filed in Japan on Oct. 27, 2011. The total contents of disclosure of the patent application of the senior filing date are to be incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

Silver plating has been long used for ornamental purposes and the like, and in recent years, with making use of its electrical characteristics and high reflectivity, it has been frequently used in the field of electrical industry, and the field of optical industry, and also in other fields, such as the field of electromagnetic wave shield and the field of sterilization coating. Especially, electroless silver plating has been more frequently used since it allows film thickness to be controlled and a plating film having a necessary thickness to be easily formed.

Electroless silver plating is broadly divided into substituting electroless silver plating and reducing electroless silver plating. Substituting electroless silver plating is relatively excellent in stability of a plating solution, and has been frequently employed in the market (For example, refer to Patent Literatures 1 and 2). However, since substituting electroless silver plating is performed in such a manner that silver plating is precipitated by a substitution reaction with an underlying metal, there is a problem that a limited kind of underlying material is used for substituting electroless silver plating.

On the other hand, reducing electroless silver plating is performed in such a manner that silver plating is precipitated on an underlying metal by making a reducing agent to be contained in a plating solution and thereby reducing a water-soluble silver compound to metallic silver, and this reducing electroless silver plating is capable of forming a good silver plating film without roughening of an underlying material and also without limiting a kind of underlying material.

Particularly, from a viewpoint of solution stability, a reducing electroless silver plating solution is made to contain a silver cyanide compound, such as silver potassium cyanide, as a water-soluble silver salt. Where this silver cyanide compound is used as a water-soluble silver salt, generally, an amount of cyanogen contained in a plating solution is more than twice as that of silver at a molar ratio.

Also, there has been proposed a method for improving the stability of a reducing electroless silver plating solution by adding a cyanide, such as potassium cyanide, wherein many free cyanides are present in the plating solution (For example, Patent Literature 3 and Non Patent Literature 1).

However, while excessive cyanogen in the prior plating solution makes it possible to control decomposition of silver in the plating solution and improve the stability as mentioned above, the excessive cyanogen dissolves a plated material which is made of, metal of nickel, copper, or the like, and excessively roughens a surface of the material, whereby a plating film having good film characteristics cannot be formed.

On the other hand, in recent years, a cyanogen-free reducing electroless silver plating solution has been also proposed (For example, Patent Literatures 4 and 5). In such cyanogen-free electroless silver plating solution, cyanogen is not present in the plating solution and therefore an underlying metal or the like is not dissolved, and accordingly, a surface thereof is not excessively roughened, however, with respect to the solution stability, the cyanogen-free electroless silver plating solution has been remarkably inferior to that of a plating solution containing cyanogen. As disclosed in Patent Literatures 4 and 5, there has been proposed a technique to improve the stability of a cyanogen-free silver plating solution by adding an additive thereto, but the cyanogen-free silver plating solution has not yet had sufficient stability. Furthermore, in such cyanogen-free electroless silver plating, the thicker a silver film becomes, the more the silver film becomes yellowish, and thus a problem arises also in view of appearance of a plating film.

PRIOR-ART DOCUMENTS Patent Document

-   PTL 1: Japanese Patent Application Laid-Open No. 2000-309875 -   PTL 2: Japanese Patent Application Laid-Open No. 2002-180259 -   PTL 3: Japanese Patent Application Laid-Open No. H5-279863 -   PTL 4: Japanese Patent Application No. 3937373 -   PTL 5: Japanese Patent Application Laid-Open No. 2003-268558

Non Patent Document

-   Non PTL 1: “Mudenkai Mekki—Kiso to Ohyo” (Electroless     Plating—Fundamentals and Applications) (edited by Electroplating     Research Society Japan, published by Nikkan Kogyo Shimbun, Ltd., pp     176-177)

SUMMARY OF THE INVENTION

The present invention is proposed in view of such actual circumstances, and aims at providing a reducing electroless silver plating solution and a reducing electroless silver plating method using the silver plating solution, the reducing electroless silver plating solution being capable of preventing decomposition of silver in the plating solution thereby to maintain the stability of the solution and also being capable of preventing excessive roughening of metal or the like as an underlying plating material thereby to form a plating film having good film characteristics and a good appearance.

The present inventors earnestly studied to achieve the above-mentioned aim, and as a result, found that control of a cyanogen concentration in a plating solution allows the stability of the plating solution to be maintained and prevents an underlying metal or the like from being excessively roughened, whereby a plating film having good film characteristics and being excellent in appearance can be formed, and the present inventors completed the present invention.

In other words, a reducing electroless silver plating solution according to the present invention comprises a water-soluble silver salt and a reducing agent, wherein cyanide ions in a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L are contained.

Furthermore, in the reducing electroless silver plating solution according to the present invention, the above-mentioned water-soluble silver salt is preferably a silver salt other than a cyanide, and the above-mentioned cyanide ions are preferably contained as an alkali metal cyanide.

Furthermore, in the reducing electroless silver plating solution according to the present invention, the above-mentioned reducing agent is preferably at least one kind or more selected from hydroxylammonium sulfate and hydroxylacetate ammonium.

Furthermore, the reducing electroless silver plating solution according to the present invention preferably has a pH of 8 to 11.

Also, a reducing electroless silver plating method according to the present invention is such that electroless silver plating is applied to a plated material, using a reducing electroless silver plating solution comprising a water-soluble silver salt and a reducing agent, wherein cyanide ions in a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L are contained.

EFFECTS OF INVENTION

The reducing electroless silver plating solution and the reducing electroless silver plating method according to the present invention are capable of preventing decomposition of silver in the plating solution thereby to make the stability of the solution good and also capable of preventing excessive roughening of metal or the like as an underlying plating material thereby to form a plating film having good film characteristics and a good appearance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates graphs showing relationships between film thickness of a plating film and plating time where different reducing agents are employed.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a specific embodiment of the reducing electroless silver plating solution according to the present invention (hereinafter, referred to as the present embodiment.) will be described in detail.

A reducing electroless silver plating solution according to the present embodiment comprises a water-soluble silver salt and a reducing agent, the silver plating solution being capable of reducing the water-soluble silver salt to metallic silver with the reducing agent and precipitating silver plating on a plated material, such as an underlying metal, thereby forming a silver plating film.

The reducing electroless silver plating solution according to the present embodiment is characterized in that, in the plating solution comprising a water-soluble silver salt and a reducing agent, cyanide ions prepared to have a predetermined concentration range are contained. Specifically, cyanide ions in a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L are contained.

A reducing electroless silver plating solution, according to the prior art, to which a water-soluble silver salt, such as silver potassium cyanide, and a cyanide, such as potassium cyanide, are added as additives is excellent in solution stability, but, excessively dissolves and roughens metal or the like as an underlying plating material composed of, for example, copper or nickel, whereby a plating film having good film characteristics has not been formed.

On the other hand, a cyanogen-free plating solution has been also developed in order to avoid such excessive dissolution of an underlying metal or the like, but, where a silver plating solution having low stability of the solution, in particular, is made to be cyanogen-free, then such plating solution causes rapid decomposition of silver contained in the plating solution, whereby the stability of the plating solution has been remarkably decreased. Furthermore, such cyanogen-free plating solution causes a yellowish plating film to be formed, whereby formation of a plating film having a good appearance has not been realized. The application of such silver plating film having a poor appearance to a LED device, for, example, has caused a lower reflectivity.

To solve this problem, the reducing electroless silver plating solution according to the present embodiment is made to contain cyanide ions having a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L in the plating solution.

Such reducing electroless silver plating solution can achieve only the advantages brought by both the prior electroless silver plating solution using a cyanide and the prior cyanogen-free electroless silver plating solution, that is, achieve higher solution stability, and can form a plating film having excellent film characteristics without excessive roughening of a plated material, such as a underlying metal. Furthermore, this reducing electroless silver plating solution allows a plating film to have a beautiful white silver color and thus to have a good appearance, and when the plating film is applied to, for example, a LED device, reflectance characteristics thereof can be improved.

Here, when an amount of cyanide ions contained is less than 0.006×10⁻³ mol/L, silver in the plating solution is decomposed, and thus the stability of the solution is decreased. Furthermore, the thicker a plating film becomes, the more the film becomes yellowish, and thus the plating film cannot be formed having a good appearance. On the other hand, when an amount of cyanide ions contained is more than 12.5×10⁻³ mol/L, cyanogen in the plating solution dissolves and excessively roughens a plated material which is composed of metal or the like as a plating underlying material, and thus even if a plating film is formed on the plated material, the plating film cannot have good film characteristics.

A source of cyanide ions is not particularly limited, but, for example, alkali metal cyanide, such as potassium cyanide or sodium cyanide, may be used. Furthermore, a silver cyanide compound, such as silver potassium cyanide, may be used as a water-soluble silver salt and made to be contained as a part or a whole of a source of cyanide ions so as to have the above-mentioned cyanide ion content.

Among these sources of cyanide ions, alkali metal cyanide is more preferably used. The use of alkali metal cyanide allows the cyanide having a cyanide ion concentration within the above-mentioned range to be appropriately and easily prepared, and allows solution stability to be maintained and a plating film having good film characteristics to be formed more efficiently and more effectively. Furthermore, also when a plating solution is continuously used, it is not necessary to add alkali metal cyanide too often, and furthermore, unlike the case with adding a silver cyanide compound, due to an increase in amount of silver contained in the plating solution, the solution stability are not decreased.

In the reducing electroless silver plating solution according to the present embodiment, the water-soluble silver salt is not particularly limited as long as it is soluble in the plating solution, and examples of the water-soluble silver salt which may be used include silver nitrate, silver oxide, silver sulfate, silver chloride, silver sulfite, silver carbonate, silver acetate, silver lactate, silver sulfosuccinate, silver sulfonate, silver sulfamate, and silver oxalate. Moreover, as mentioned above, a silver cyanide compound, such as silver potassium cyanide, may be used as a water-soluble silver salt. These water-soluble silver salts may be used alone or two or more kinds thereof may be used in combination.

The water-soluble silver salt is preferably contained in a silver concentration of 0.1 g/L to 10 g/L (0.9×10⁻³ mol/L to 90×10⁻³ mol/L), more preferably 0.1 g/L to 3.0 g/L (0.9×10⁻³ mol/L to 30×10⁻³ mol/L). When a water-soluble silver salt is contained in a silver concentration within a range of 0.1 g/L to 10 g/L, a precipitation rate of silver plating can be higher, and also a plating solution having higher stability can be achieved.

Moreover, where a silver cyanide compound is used as a water-soluble silver salt, a silver cyanide compound to be added or a cyanide as an additive contained together with the silver cyanide compound are made to be contained in a cyanide ion concentration within the above-mentioned range, that is a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L.

The reducing agent is not particularly limited as long as it is capable of reducing a water-soluble silver salt contained in a plating solution to metallic silver and also is water-soluble, for example, the reducing agent may be used including hydrazine and its derivative, a formaldehyde compound, hydroxylammonium salt, saccharides, Rochelle salt, a boron hydride compound, hypophosphite, DMAB (dimethyl amine boranes), and ascorbic acid and the like. These reducing agents may be used alone or two or more kinds thereof may be used in combination.

Among these reducing agents, particularly hydroxylammonium salts, such as hydroxylammonium sulfate and hydroxylacetate ammonium, are preferably used. The reason for this is that these reducing agents make it possible to easily change plating thickness by controlling plating time, and to form a plating film having a desired plating thickness and good film characteristics with a film-thickening treatment or the like.

More specifically, in the reducing electroless silver plating solution according to the present embodiment, as mentioned above, in order not to excessively dissolve an underlying metal or the like, cyanide ions are controlled to be in a predetermined concentration range. On the other hand, where a reducing agent having higher reducing power, such as DMAB, is used, it is possible to cause decomposition of silver in a plating solution. In this respect, the use of hydroxylammonium salt, such as hydroxylammonium sulfate or hydroxylacetate ammonium, prevents decomposition of silver in a plating solution, and thus can lead to a stable reduction reaction.

Also, where a reducing agent having weaker reducing power, such as hydrazine or formaldehyde, is used, a silver precipitation rate is slower. Therefore, a portion of the surface of an underlying metal or the like on which silver is not precipitated is present for a long time, and thus cyanogen in a plating solution may act on the portion and excessively dissolve it. Also in this respect, the use of hydroxylamines, such as hydroxylammonium sulfate or hydroxylacetate ammonium allows silver plating to be certainly precipitated by a reduction reaction without dissolution of an underlying metal or the like, and therefore, by control of plating time, the underlying metal or the like can be coated with silver plating with a desired film thickness, and a plating film having good film characteristics can be formed.

Thus, the use of hydroxylammonium sulfate or hydroxylacetate ammonium allows a plating film with a desired film thickness to be easily formed and solution stability to be higher, and prevents excessive dissolution of an underlying metal or the like due to cyanogen, whereby a plating film having good film characteristics can be more effectively formed.

A reducing agent is preferably contained in a concentration of, for example, 0.006 mol/L to 0.12 mol/L, more preferably 0.006 mol/L to 0.03 mol/L. When a reducing agent is contained in a concentration of less than 0.006 mol/L, it is possible that a water-soluble silver salt in a plating solution cannot be reduced to metallic silver and sufficient silver plating cannot be precipitated. On the other hand, when a reducing agent is contained in a concentration of more than 0.12 mol/L, the stability of a plating solution is adversely affected and it is not economically preferable.

The reducing electroless silver plating solution according to the present embodiment may be used at a liquid temperature within a range of 0 to 80 degrees C., and, particularly, the use of the plating solution at a temperature of approximately 30 to 60 degrees C. allows the stability of the plating solution to be higher. When the temperature of the plating solution is too low, silver is precipitated at a slow rate, and it takes a long time to obtain a predetermined amount of silver precipitate. On the other hand, when the temperature of the plating solution is too high, it is easily to cause the loss of a reducing agent due to its autolysis reaction and the decrease in the stability in plating solution.

Furthermore, the reducing electroless silver plating solution may be used when the solution has a pH of 2 to 14, but, since cyanide ions are contained in a predetermined concentration as mentioned above, in particular, the reducing electroless silver plating solution preferably has a pH of 8 to 11. When the plating solution has a pH of not less than 8, generation of cyanogen gas can be effectively controlled and the plating solution can be safely used without adverse effects on environment. Furthermore, the stability of the plating solution can be higher. Also, the plating solution having a pH of not more than 11 allows the stability of the plating solution and the film characteristics of a plating film to be improved.

The pH adjustment for a plating solution is carried out in such a manner that, when pH is lowered, an acid is usually used which has the same kind of anion portion as an anion portion of a water-soluble silver salt, for example, sulfuric acid in the case of using silver sulfate as a water-soluble silver salt, or nitric acid in the case of using silver nitrate as a water-soluble silver salt. On the other hand, when pH is raised, alkali metal hydroxide, such as sodium hydroxide, ammonia, or the like is used.

Moreover, with respect to the reducing electroless silver plating solution according to the present embodiment, a complexing agent may be added as needed. The complexing agent is not particularly limited, and examples of the complexing agent include sulfite, succinimide, hydantoin derivatives, ethylenediamine, and ethylenediaminetetraacetic acid (EDTA). These complexing agents may be used alone or two or more kinds thereof may be used in combination.

An amount of additive of the complexing agent depends on its kind and is not particularly limited, but preferably approximately 1 g/L to 100 g/L. The complexing agent having such concentration allows a good precipitation rate of silver plating to be achieved and the plating solution to have more excellent stability.

Moreover, an additive, such as a well-known surface active agent, a pH adjuster, a buffer, a lubricant, and a stress relaxation agent, may be mixed, as needed.

A plating method using the reducing electroless silver plating solution having the above-mentioned characteristics is, for example, such that a plated material is immersed in the reducing electroless silver plating solution having the liquid temperature and the pH value each adjusted as mentioned above, whereby silver-plating is applied to the material. Also, the reducing electroless silver plating solution is sprayed or applied to a plated material, whereby the plated material is brought into contact with the plating solution to undergo a plating treatment.

As mentioned above, the electroless silver plating method operates in such a manner that, mainly, a water-soluble silver salt is reduced to metallic silver with the reducing agent contained in the plating solution, and silver plating is precipitated on a plated material, such as metal, as an underlying plating material, whereby a plating film is formed.

The plated material to form an electroless silver plating film is not particularly limited, and metallic materials, such as copper and nickel, various kinds of other conductive materials and non-conductivity materials, and the like may be applied. Where a metallic material is used as a plated material, a pretreatment, such as a degreasing treatment, is applied thereto in accordance with a usual method, and then the plated material is directly immersed in a plating solution.

Moreover, in the case of plating a nonmetallic material, such as ceramics and plastics, a pretreatment, such as a degreasing treatment, is applied to the plated material, and then an activation treatment is applied thereto, followed by immersion thereof in the plating solution. It is beneficial to perform the activation treatment in accordance with a usual method, and, for example, the activation treatment is performed using a palladium catalyst (catalyst-accelerator process, sensitizer-activator process, or the like), a silver catalyst, a copper catalyst, or the like, in accordance with well-known conditions.

As mentioned above, the reducing electroless silver plating solution according to the present embodiment comprises a water-soluble silver salt and a reducing agent, wherein cyanide ions are contained at a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L. Such reducing electroless silver plating solution is capable of effectively preventing the decomposition of silver contained in the silver plating solution thereby to achieve good stability of the solution and also capable of preventing excessive roughening of a plated material, such as an underlying metal, thereby to form a plating film having good film characteristics and a good appearance.

EXAMPLES

Hereinafter, specific Examples of the present invention will be described. It should be noted that the present invention is not limited to any of the following Examples.

Plating Solution Stability and Plating Film Evaluation

As shown below, reducing electroless silver plating solutions each were adjusted.

Example 1

There was prepared an aqueous solution containing a silver nitrate having a silver concentration of 9.0×10⁻³ mol/L (1.0 g/L), a hydroxylammonium salt (hydroxylammonium sulfate) in a concentration of 1.24×10⁻³ mol/L as a reducing agent, and EDTA in a concentration of 0.15 mol/L (50 g/L) as a complexing agent, and furthermore, potassium cyanide in a concentration of 1 mg/L was added thereto, whereby the aqueous solution is made to have a cyanide ion concentration of 0.006×10⁻³ mol/L in a plating solution, and the aqueous solution is adjusted using caustic soda to have a pH of 9.0, whereby a reducing electroless silver plating solution was prepared.

Example 2

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that potassium cyanide in a concentration of 300 mg/L was added, thereby allowing the plating solution to have a cyanide ion concentration of 1.8×10⁻³ mol/L.

Example 3

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that potassium cyanide in a concentration of 500 mg/L was added, thereby allowing the plating solution to have a cyanide ion concentration of 3.0×10⁻³ mol/L.

Example 4

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that potassium cyanide in a concentration of 1000 mg/L was added, thereby allowing the plating solution to have a cyanide ion concentration of 6.5×10⁻³ mol/L.

Example 5

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that potassium cyanide in a concentration of 2000 mg/L was added, thereby allowing the plating solution to have a cyanide ion concentration of 12.5×10⁻³ mol/L.

Comparative Example 1

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that a silver potassium cyanide having a silver concentration of 9.0×10⁻³ mol/L (1.0 g/L) was added in place of silver nitrate, and furthermore a potassium cyanide in a concentration of 300 mg/L was added, whereby the plating solution had a cyanide ion concentration of 19.8×10⁻³ mol/L.

Comparative Example 2

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that a silver potassium cyanide having a silver concentration of 9.0×10⁻³ mol/L (1.0 g/L) was added in place of silver nitrate, and potassium cyanide was not added, whereby the plating solution had a cyanide ion concentration of 18.0×10⁻³ mol/L.

Comparative Example 3

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that potassium cyanide was not added. In other words, a cyanogen-free reducing electroless silver plating solution was prepared.

Comparative Example 4

A reducing electroless silver plating solution was prepared in the same manner as in Example 1, except that a potassium cyanide in a concentration of 2100 mg/L was added, thereby allowing the plating solution to have a cyanide ion concentration of 13.0×10⁻³ mol/L.

Next, using each of the reducing electroless silver plating solutions prepared as mentioned above, reducing electroless silver plating was applied to a BGA substrate (manufactured by C. Uyemura & Co., Ltd.) as a plated material.

It should be noted that, prior to the reducing electroless silver plating treatment, each step shown in the following Table 1 was performed in order as a pretreatment. In other words, a cleaner treatment (degreasing) using ACL-738 (manufactured by C. Uyemura & Co., Ltd.) was applied to a BGA substrate as a plated material, and then soft etching was performed using a sodium persulfate solution (SPS) in a concentration of 100 g/L. Next, etching residues were removed using a 10% sulfuric acid (H₂SO₄) solution (acid pickling), and pre-dipping was performed using a 3% sulfuric acid solution, and then a Pd catalyst treatment was performed using MNK-4 (manufactured by C. Uyemura & Co., Ltd.) (catalyst treatment). After that, by using an electroless nickel solution NPR-4 (manufactured by C. Uyemura & Co., Ltd.) and an electroless palladium solution TPD-30 (manufactured by C. Uyemura & Co., Ltd.), a metallic film was formed as an underlying material.

TABLE 1 Treatment Step Chemicals Temperature time (min) Cleaner ACL-738 50 5 Soft-etching SPS 100 g/L 25 1 Acid pickling 10% H₂SO₄ r.t. 1 Pre-dipping 3% H₂SO₄ r.t. 1 Catalyst MNK-4 30 2 Electroless Ni NPR-4 80 25 Solution Electroless Pb TPD-30 55 2 Solution

The electroless silver plating treatment was performed in such a manner that a plated material was immersed in the above-mentioned reducing electroless silver plating solution at 60 degrees C. for 20 minutes. After performing the plating treatment, the plating solution was left for 100 hours with keeping the temperature at 60 degrees C. Then, the stability of the plating solution was evaluated by whether the plating solution was self-decomposed or not, and also the solder joint strength and the appearance of a plating film were evaluated. The following Table 2 shows evaluation results.

It should be noted that, in order to evaluate the solder joint strength of a plating film, a reflow treatment was performed once at 240 degrees C., and the joint strength was evaluated in such a manner that, when a solder fracture mode was found in 16 or more among 20 solder joints, the joint strength was evaluated to be good (◯), on the other hand, when a solder fracture mode was found in less than 16 solder joints, the joint strength was evaluated to be poor (X). Furthermore, the appearance of a silver plating film was evaluated by visually observing the appearance of the plating film having a thickness of 0.5 micrometer.

TABLE 2 Solder Bath joint Cyanogen concentration in solution stability strength Appearance Example 1 0.006 × 10⁻³ mol/L not ◯ white silver KCN: 1 mg/L(0.015 × 10⁻³ mol/L) decomposed Example 2 1.8 × 10⁻³ mol/L not ◯ white silver KCN: 300 mg/L(4.6 × 10⁻³ mol/L) decomposed Example 3 3.0 × 10⁻³ mol/L not ◯ white silver KCN: 500 mg/L(7.5 × 10⁻³ mol/L) decomposed Example 4 6.5 × 10⁻³ mol/L not ◯ white silver KCN: 1000 mg/L(16.0 × 10⁻³ mol/L) decomposed Example 5 12.5 × 10⁻³ mol/L not ◯ white silver KCN: 2000 mg/L(31.0 × 10⁻³ mol/L) decomposed Comparative 19.8 × 10⁻³ mol/L not X white silver Example 1 decomposed Comparative 18.0 × 10⁻³ mol/L not X white silver Example 2 decomposed Comparative 0 precipitated ◯ yellow Example 3 in beaker silver Comparative 13.0 × 10⁻³ mol/L not X white silver Example 4 KCN: 2100 mg/L(33.0 × 10⁻³ mol/L) decomposed

As shown in Table 2, in the reducing electroless silver plating solutions in Examples 1 to 5 each of which was controlled to have a cyanide ion concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L, silver contained in the plating solution was not decomposed and good solution stability was achieved. Furthermore, in each of Examples 1 to 5, a plating film of higher solder joint strength and having good film characteristics was formed. Furthermore, the formed plating film was white-silver-colored and had a beautiful appearance.

On the other hand, in the reducing electroless silver plating solutions in Comparative Examples 1, 2, and 4 which were controlled to have a cyanide ion concentration of 19.8×10⁻³ mol/L, 18.0×10⁻³ mol/L, and 13.0×10⁻³ mol/L, respectively, excessive free-cyanide in the plating solution prevented silver from being decomposed while a plating film of lower solder joint strength and having poor film characteristics was formed. The reason for this may be that, due to the excessive free-cyanide present in the plating solution, an underlying metal was excessively dissolved and roughened.

Also, in the reducing electroless silver plating solution in Comparative Example 3 which was cyanogen-free, a plating film of higher solder joint strength and having good film characteristics was formed. On the other hand, silver in the plating solution was decomposed due to the cyanogen-free, and the stability of the plating solution was not secured. Furthermore, where the reducing electroless silver plating solution in Comparative Example 3 was used, a yellowish plating film, that is, a yellow silver plating film was formed, thereby having an extremely poor appearance.

From the above-mentioned results, it was found that, when the plating solution had a cyanide ion concentration in a range of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L, decomposition of silver was controlled and the stability of the solution was maintained, and moreover, beautiful and white-silver-colored silver was precipitated and thus a film having an excellent appearance was formed. Furthermore, it was found that a cyanide ion concentration in the above-mentioned range prevented a metallic material or the like as an underlying plating material from being excessively roughened, thereby allowing a plating film having good film characteristics to be formed.

Reducing Agent

Next, it was examined what type of a reducing agent is preferable for the reducing electroless silver plating solution which is controlled to have a cyanide ion concentration in a range of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L, as in the above-mentioned Examples 1 to 5. It should be noted that experiments were conducted using the composition employed in Example 2 as a basic composition.

Example 6

A reducing electroless silver plating solution having a cyanide ion concentration of 0.006×10⁻³ mol/L was prepared in the same manner as in Example 2, except that hydrazine (hydrazine sulfate) in a concentration of 1.24×10⁻³ mol/L was added as a reducing agent.

Example 7

A reducing electroless silver plating solution having a cyanide ion concentration of 0.006×10⁻³ mol/L was prepared in the same manner as in Example 2, except that formaldehyde in a concentration of 1.24×10⁻³ mol/L was added as a reducing agent.

Using the reducing electroless silver plating solutions prepared in the above-mentioned Examples 2, 6, and 7, the correlation between thickness of a silver plating film and plating time was examined. The graphs in FIG. 1 show the results of the measurement of film thickness with respect to plating time when the different reducing agents were used.

As shown in FIG. 1, it is understood that, in the electroless silver plating solution of Example 2 using a hydroxylammonium salt as a reducing agent, as a plating time is extended, the resulting thickness of a silver plating film is approximately linearly increased, and it is understood that a reduction reaction allows silver to be precipitated, whereby a film is formed. On the other hand, it is understood that, in Examples 6 and 7 using hydrazine and formaldehyde as reducing agents, respectively, once a predetermined time passes, the thickness is not much increased after that. The reason for this may be that hydrazine and formaldehyde allow silver to be precipitated by a reduction reaction while a factor resulting from a substitution reaction greatly works, whereby an underlying metal is slightly dissolved.

Hence, it was found that, in the reducing electroless silver plating solution which is controlled to have a cyanide ion concentration in a range of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L, the use of a hydroxylammonium salt, such as hydroxylammonium sulfate or hydroxylacetate ammonium, as a reducing agent allows a reduction reaction to proceed more effectively and dissolution of an underlying metal by a substitution reaction not to be caused, whereby a plating film having good film characteristics can be formed. 

1. A reducing electroless silver plating solution, comprising: a water-soluble silver salt and a reducing agent, wherein cyanide ions in a concentration of 0.006×10⁻³ mol/L to 12.5×10⁻³ mol/L are contained.
 2. The reducing electroless silver plating solution according to claim 1, wherein the above-mentioned water-soluble silver salt is a silver salt other than a cyanide, and the above-mentioned cyanide ions are contained as an alkali metal cyanide.
 3. The reducing electroless silver plating solution according to claim 1, wherein the above-mentioned reducing agent is at least one kind or more selected from hydroxylammonium sulfate and hydroxylacetate ammonium.
 4. The reducing electroless silver plating solution according to claim 1, having a pH of 8 to
 11. 5. A reducing electroless silver plating method, wherein, using the reducing electroless silver plating solution according to claim 1, electroless silver plating is applied to a plated material. 